Responses
I think this sounds like the backstory (chapter 34) of this:
Princess Shuden
https://www.mangago.me/read-manga/princess_shuden/
Yes!!! Thank you!!!
Three brothers were poisoned by their grandfather and the antidote was in the cake. The two brothers ate the cake without giving any to the youngest. They have no knowledge of the poisoning. The grandfather took one boy and became his heir. After several years, the other brother became his enemy. Pleaseeee help me find it. I can't sleep thinking about its title.